US 8175197 B2 Abstract A system includes a correlation module and a control module. The correlation module is configured to generate correlation values based on a correlation of modulated signals with a plurality of preamble sequences and generate correlation values. The modulated signals include sub-carriers modulated using orthogonal frequency domain multiplexing (OFDM). The control module is configured to select a largest correlation value from the correlation values and detect one of the preamble sequences in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a first predetermined threshold. The control module divides N of the sub-carriers into L bands in response to a channel gain of the sub-carriers not being substantially the same for all of the sub-carriers, where N and L are integers greater than 1, and where each of the L bands includes N/L of the sub-carriers.
Claims(52) 1. A system, comprising:
a correlation module configured to generate correlation values based on a correlation of modulated signals with a plurality of preamble sequences, wherein the modulated signals include sub-carriers modulated using orthogonal frequency domain multiplexing (OFDM); and
a control module configured to
select a largest correlation value from the correlation values, and
detect one of the preamble sequences in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a first predetermined threshold,
wherein the control module divides N of the sub-carriers into L bands in response to a channel gain of the sub-carriers not being substantially the same for all of the sub-carriers, where N and L are integers greater than 1, and where each of the L bands includes N/L of the sub-carriers.
2. The system of
3. The system of
4. The system of
5. The system of
^{th }one of the sub-carriers is modulated with a preamble bit, where P is an integer greater than or equal to 1.6. The system of
7. The system of
8. The system of
9. The system of
10. The system of
11. The system of
12. A physical layer module (PHY) comprising:
the system of
a transceiver module configured to
communicate with the correlation module and the control module; and
receive the modulated signals.
13. A network device comprising:
the PHY of
at least one antenna configured to communicate with the transceiver module.
14. The system of
correlate symbols in every P
^{th }one of the N/L of the sub-carriers in each of the L bands with corresponding symbols in each of the preamble sequences; andgenerate intra-band correlation values for each band for each of the preamble sequences,
where P is an integer greater than or equal to 1.
15. The system of
generate a band correlation value for each of the L bands and for each of the preamble sequences by adding the intra-band correlation values;
generate a magnitude of each of the band correlation value; and
generate the correlation values by adding the magnitude of each of the band correlation value for each of the preamble sequences.
16. A method, comprising:
receiving modulated signals, wherein the modulated signals include sub-carriers modulated using orthogonal frequency domain multiplexing (OFDM);
dividing N of the sub-carriers into L bands in response to a channel gain of the sub-carriers not being substantially the same for all of the sub-carriers, where N and L are integers greater than 1, and where each of the L bands includes N/L of the sub-carriers;
correlating the modulated signals with a plurality of preamble sequences;
generating correlation values based on the correlating;
selecting a largest correlation value from the correlation values; and
detecting one of the preamble sequences in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a first predetermined threshold.
17. The method of
18. The method of
19. The method of
^{th }one of the sub-carriers is modulated with a preamble bit, where P is an integer greater than or equal to 1, and wherein the sub-carriers have a random channel phase.20. The method of
21. The method of
22. The method of
23. The method of
24. The method of
25. The method of
correlating symbols in every P
^{th }one of the N/L of the sub-carriers in each of the L bands with corresponding symbols in each of the preamble sequences; andgenerating intra-band correlation values for each band for each of the preamble sequences,
where P is an integer greater than or equal to 1.
26. The method of
generating a band correlation value for each of the L bands and for each of the preamble sequences by adding the intra-band correlation values;
generating a magnitude of each of the band correlation value; and
generating the correlation values by adding the magnitude of each of the band correlation value for each of the preamble sequences.
27. A system, comprising:
correlation means for generating correlation values based on a correlation of modulated signals with a plurality of preamble sequences, wherein the modulated signals include sub-carriers modulated using orthogonal frequency domain multiplexing (OFDM); and
control means for
selecting a largest correlation value from the correlation values, and
detecting one of the preamble sequences in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a first predetermined threshold,
wherein the control means divides N of the sub-carriers into L bands in response to a channel gain of the sub-carriers not being substantially the same for all of the sub-carriers, where N and L are integers greater than 1, and where each of the L bands includes N/L of the sub-carriers.
28. The system of
29. The system of
30. The system of
31. The system of
^{th }one of the sub-carriers is modulated with a preamble bit, where P is an integer greater than or equal to 1.32. The system of
33. The system of
34. The system of
35. The system of
36. The system of
37. The system of
38. A physical layer means (PHY) comprising:
the system of
transceiver means for communicating with the correlation means and the control means and that receives the modulated signals.
39. A network device comprising:
the PHY means of
at least one antenna means for communicating with the transceiver means.
40. The system of
correlates symbols in every P
^{th }one of the N/L of the sub-carriers in each of the L bands with corresponding symbols in each of the preamble sequences, andgenerates intra-band correlation values for each band for each of the preamble sequences,
where P is an integer greater than or equal to 1.
41. The system of
generates a band correlation value for each of the L bands and for each of the preamble sequences by adding the intra-band correlation values;
generates a magnitude of each of the band correlation value; and
generates the correlation values by adding the magnitude of each of the band correlation value for each of the preamble sequences.
42. A computer program stored on a non-transitory computer-readable medium and executable by a processor, the computer program comprising instructions for:
receiving modulated signals, wherein the modulated signals include sub-carriers modulated using orthogonal frequency domain multiplexing (OFDM);
dividing N of the sub-carriers into L bands in response to a channel gain of the sub-carriers not being substantially the same for all of the sub-carriers, where N and L are integers greater than 1, and where each of the L bands includes N/L of the sub-carriers;
correlating the modulated signals with a plurality of preamble sequences;
generating correlation values based on the correlating;
selecting a largest correlation value from the correlation values; and
detecting one of the preamble sequences in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a first predetermined threshold.
43. The computer program of
44. The computer program of
45. The computer program of
^{th }one of the sub-carriers is modulated with a preamble bit, where P is an integer greater than or equal to 1, and wherein the sub-carriers have a random channel phase.46. The computer program of
47. The computer program of
48. The computer program of
49. The computer program of
50. The computer program of
51. The computer program of
correlating symbols in every P
^{th }one of the N/L of the sub-carriers in each of the L bands with corresponding symbols in each of the preamble sequences; andgenerating intra-band correlation values for each band for each of the preamble sequences,
where P is an integer greater than or equal to 1.
52. The computer program of
generating a band correlation value for each of the L bands and for each of the preamble sequences by adding the intra-band correlation values,
generating a magnitude of each of the band correlation value, and
generating the correlation values by adding the magnitude of each of the band correlation value for each of the preamble sequences.
Description This application is a continuation of U.S. Ser. No. 11/648,735, filed Dec. 29, 2006, which application claims the benefit of U.S. Provisional Application No. 60/783,300, filed on Mar. 17, 2006, U.S. Provisional Application No. 60/792,508, filed on Apr. 17, 2006, U.S. Provisional Application No. 60/809,733, filed on May 31, 2006, and U.S. Provisional Application No. 60/826,392, Sep. 21, 2006. The disclosures of the above applications are incorporated herein by reference in their entirety. The present disclosure relates to communication systems, and more particularly to detecting preamble sequences in systems using orthogonal frequency domain multiplexing (OFDM). The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. Referring now to The information source The waveform output by the modulator The demodulator Communication systems use different modulation schemes to modulate and transmit data. For example, a radio frequency (RF) carrier may be modulated using techniques such as frequency modulation, phase modulation, etc. In wireline communication systems, a transmitted signal generally travels along a path in a transmission line between a transmitter and a receiver. In wireless communication systems, however, a transmitted signal may travel along multiple paths. This is because the transmitted signal may be reflected and deflected by objects such as buildings, towers, airplanes, cars, etc., before the transmitted signal reaches a receiver. Each path may be of different length. Thus, the receiver may receive multiple versions of the transmitted signal. The multiple versions may interfere with each other causing inter symbol interference (ISI). Thus, retrieving original data from the transmitted signal may be difficult. To alleviate this problem, wireless communication systems often use a modulation scheme called orthogonal frequency division multiplexing (OFDM). In OFDM, a wideband carrier signal is converted into a series of independent narrowband sub-carrier signals that are adjacent to each other in frequency domain. Data to be transmitted is split into multiple parallel data streams. Each data stream is modulated using a sub-carrier. A channel over which the modulated data is transmitted comprises a sum of the narrowband sub-carrier signals, which may overlap. When each sub-carrier closely resembles a rectangular pulse, modulation can be easily performed by Inverse Discrete Fourier Transform (IDFT), which can be efficiently implemented as an Inverse Fast Fourier Transform (IFFT). When IFFT is used, the spacing of sub-carriers in the frequency domain is such that when the receiver processes a received signal at a particular frequency, all other signals are nearly zero at that frequency, and ISI is avoided. This property is called orthogonality, and hence the modulation scheme is called orthogonal frequency division multiplexing (OFDM). Referring now to Specifically, each BS may transmit data using orthogonal frequency division multiplexing access (OFDMA) system. Each BS may transmit data typically in three segments: SEG Relative motion between MS and BS may cause Doppler shifts in signals received by the MS. This can be problematic since systems using OFDMA are inherently sensitive to carrier frequency offsets (CFO). Therefore, pilot tones are generally used for channel estimation refinement. For example, some of the sub-carriers may be designated as pilot tones for correcting residual frequency offset errors. Additionally, the PHY module According to the I.E.E.E. standard 802.16e, which is incorporated herein by reference in its entirety, a first symbol in the data frame transmitted by the BS is a preamble symbol from a preamble sequence. The preamble sequence typically contains an identifier called IDcell, which is a cell ID of the BS, and segment information. The BS selects the preamble sequence based on the IDcell and the segment number of the BS. Each BS may select different preamble sequences. Additionally, each BS may select preamble sequences that are distinct among the segments of that BS. The BS modulates multiple sub-carriers with the selected preamble sequence. Thereafter, the BS performs IFFT, adds a cyclic prefix, and transmits a data frame. The MS uses the cyclic prefix to perform symbol timing and fractional carrier frequency synchronization. Unless the MS knows the preamble sequence, however, the MS cannot associate itself to a particular segment of a particular BS. A system comprises a differential demodulation module and a correlation module. The differential demodulation module differentially demodulates modulated signals to generate differentially demodulated signals. The correlation module correlates the differentially demodulated signals with derived preamble sequences and generates correlation values. In another feature, the modulated signals include sub-carriers that are modulated using orthogonal frequency domain multiplexing (OFDM). In another feature, every P In another feature, every P In another feature, each one of the correlation values has a real part and an imaginary part. In another feature, the system further comprises a control module that selects one of the correlation values having a largest real part and that detects a preamble sequence in the modulated signals upon determining that the largest real part is greater than or equal to a predetermined threshold. The predetermined threshold is based on the signal strength of the modulated signals. The control module identifies a segment of a base station that transmitted the modulated signals based on the preamble sequence. In another feature, every P In another feature, the system further comprises a control module that selects a largest correlation value from the correlation values and that detects a preamble sequence in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a predetermined threshold. The predetermined threshold is based on the signal strength of the modulated signals. The control module identifies a segment of a base station that transmitted the modulated signals based on the preamble sequence. In another feature, the differential demodulation module generates the differentially demodulated signals by multiplying a Q In another feature, the derived preamble sequences are derived from preamble sequences, and wherein each of the preamble sequences is different from others of the preamble sequences. In another feature, each bit of one of the derived preamble sequences has a first state when a corresponding bit and a bit adjacent to the corresponding bit in a corresponding one of the preamble sequences have opposite states, and the each bit has a second state when the corresponding bit and the bit adjacent to the corresponding bit have the same state. In another feature, the derived preamble sequences are stored in one of the correlation module and the control module. The derived preamble sequences have a cross-correlation value that is less than or equal to a predetermined cross-correlation threshold. The predetermined cross-correlation threshold is less than approximately 0.2 for an orthogonal frequency domain multiplexing (OFDM) system using a 1024 fast Fourier transform (FFT) mode. In another feature, the modulated signals include a fractional carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. In another feature, the modulated signals include an integer carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. The correlation module correlates the derived preamble sequences with the differentially demodulated signals having the integer CFO. In another feature, the modulated signals include a linearly increasing phase offset generated by a symbol timing offset. In another feature, the control module calculates a symbol timing offset by multiplying a phase angle of the largest correlation value by (N/2πP), where N is a number of sub-carriers in an N fast Fourier transform (FFT) mode, and where N is an integer greater than 1. N is one of 128, 512, 1024, and 2048. In another feature, a physical layer module (PHY) comprises the system and further comprises a transceiver module that communicates with the differential demodulation module and that receives the modulated signals. In another feature, a network device comprises the PHY and further comprises at least one antenna that communicates with the transceiver module. In still other features, a method comprises differentially demodulating modulated signals, generating differentially demodulated signals from the modulated signals, correlating the differentially demodulated signals with derived preamble sequences, and generating correlation values based on the correlating. In another feature, the method further comprises differentially demodulating sub-carriers that are included in the modulated signals and that are modulated using orthogonal frequency domain multiplexing (OFDM), wherein every P In another feature, the method further comprises differentially demodulating the sub-carriers, wherein the every P In another feature, the method further comprises determining a real part and an imaginary part of each one of the correlation values. In another feature, the method further comprises selecting one of the correlation values having a largest real part and detecting a preamble sequence in the modulated signals upon determining that the largest real part is greater than or equal to a predetermined threshold. The method further comprises determining the predetermined threshold based on the signal strength of the modulated signals. The method further comprises identifying a segment of a base station that transmitted the modulated signals based on the preamble sequence. In another feature, the method further comprises differentially demodulating the sub-carriers, wherein the every P In another feature, the method further comprises selecting a largest correlation value from the correlation values and detecting a preamble sequence in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a predetermined threshold. The method further comprises determining the predetermined threshold based on the signal strength of the modulated signals. The method further comprises identifying a segment of a base station that transmitted the modulated signals based on the preamble sequence. In another feature, the method further comprises generating the differentially demodulated signals by multiplying a Q In another feature, the method further comprises deriving the derived preamble sequences from preamble sequences, wherein each of the preamble sequences is different from others of the preamble sequences, and wherein the derived preamble sequences have a cross-correlation value that is less than or equal to a predetermined cross-correlation threshold. The method further comprises determining that the predetermined cross-correlation threshold is less than approximately 0.2 for an orthogonal frequency domain multiplexing (OFDM) method using a 1024 fast Fourier transform (FFT) mode. In another feature, the method further comprises generating each bit of one of the derived preamble sequences having a first state when a corresponding bit and a bit adjacent to the corresponding bit in a corresponding one of the preamble sequences have opposite states, and generating the each bit having a second state when the corresponding bit and the bit adjacent to the corresponding bit have the same state. In another feature, the method further comprises storing the derived preamble sequences. In another feature, the method further comprises differentially demodulating the modulated signals having a fractional carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. In another feature, the method further comprises differentially demodulating the modulated signals having an integer carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. The method further comprises correlating the derived preamble sequences with the differentially demodulated signals having the integer CFO. In another feature, the method further comprises differentially demodulating the modulated signals having a linearly increasing phase offset generated by a symbol timing offset. In another feature, the method further comprises calculating a symbol timing offset by multiplying a phase angle of the largest correlation value by (N/2πP), where N is a number of sub-carriers in an N fast Fourier transform (FFT) mode, and where N is an integer greater than 1. N is one of 128, 512, 1024, and 2048. In still other features, a system comprises differential demodulation means for differentially demodulating modulated signals to generate differentially demodulated signals and correlation means for correlating the differentially demodulated signals with derived preamble sequences and generating correlation values. In another feature, the modulated signals include sub-carriers that are modulated using orthogonal frequency domain multiplexing (OFDM). In another feature, every P In another feature, every P In another feature, each one of the correlation values has a real part and an imaginary part. In another feature, the system further comprises control means for selecting one of the correlation values having a largest real part and detecting a preamble sequence in the modulated signals upon determining that the largest real part is greater than or equal to a predetermined threshold. The predetermined threshold is based on the signal strength of the modulated signals. The control means identifies a segment of a base station that transmitted the modulated signals based on the preamble sequence. In another feature, every P In another feature, the system further comprises control means for selecting a largest correlation value from the correlation values and detecting a preamble sequence in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a predetermined threshold. The predetermined threshold is based on the signal strength of the modulated signals. The control means identifies a segment of a base station that transmitted the modulated signals based on the preamble sequence. In another feature, the differential demodulation means generates the differentially demodulated signals by multiplying a Q In another feature, the derived preamble sequences are derived from preamble sequences, and wherein each of the preamble sequences is different from others of the preamble sequences. In another feature, each bit of one of the derived preamble sequences has a first state when a corresponding bit and a bit adjacent to the corresponding bit in a corresponding one of the preamble sequences have opposite states, and the each bit has a second state when the corresponding bit and the bit adjacent to the corresponding bit have the same state. In another feature, the derived preamble sequences are stored in one of the correlation means and the control means. In another feature, the derived preamble sequences have a cross-correlation value that is less than or equal to a predetermined cross-correlation threshold. The predetermined cross-correlation threshold is less than approximately 0.2 for an orthogonal frequency domain multiplexing (OFDM) system using a 1024 fast Fourier transform (FFT) mode. In another feature, the modulated signals include a fractional carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. In another feature, the modulated signals include an integer carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. The correlation means correlates the derived preamble sequences with the differentially demodulated signals having the integer CFO. In another feature, the modulated signals include a linearly increasing phase offset generated by a symbol timing offset. In another feature, the control means calculates a symbol timing offset by multiplying a phase angle of the largest correlation value by (N/2πP), where N is a number of sub-carriers in an N fast Fourier transform (FFT) mode, and where N is an integer greater than 1. N is one of 128, 512, 1024, and 2048. In another feature, a physical layer means (PHY) for communicating comprises the system and further comprises transceiver means for communicating with the differential demodulation means and receiving the modulated signals. In another feature, a network device comprises the PHY means and further comprises at least one antenna means for communicating with the transceiver means. In still other features, a computer program executed by a processor comprises differentially demodulating modulated signals, generating differentially demodulated signals from the modulated signals, correlating the differentially demodulated signals with derived preamble sequences, and generating correlation values based on the correlating. In another feature, the computer program further comprises differentially demodulating sub-carriers that are included in the modulated signals and that are modulated using orthogonal frequency domain multiplexing (OFDM), wherein every P In another feature, the computer program further comprises differentially demodulating the sub-carriers, wherein the every P In another feature, the computer program further comprises determining a real part and an imaginary part of each one of the correlation values. In another feature, the computer program further comprises selecting one of the correlation values having a largest real part and detecting a preamble sequence in the modulated signals upon determining that the largest real part is greater than or equal to a predetermined threshold. The computer program further comprises determining the predetermined threshold based on the signal strength of the modulated signals. The computer program further comprises identifying a segment of a base station that transmitted the modulated signals based on the preamble sequence. In another feature, the computer program further comprises differentially demodulating the sub-carriers, wherein the every P In another feature, the computer program further comprises selecting a largest correlation value from the correlation values and detecting a preamble sequence in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a predetermined threshold. The computer program further comprises determining the predetermined threshold based on the signal strength of the modulated signals. The computer program further comprises identifying a segment of a base station that transmitted the modulated signals based on the preamble sequence. In another feature, the computer program further comprises generating the differentially demodulated signals by multiplying a Q In another feature, the computer program further comprises deriving the derived preamble sequences from preamble sequences, wherein each of the preamble sequences is different from others of the preamble sequences, and wherein the derived preamble sequences have a cross-correlation value that is less than or equal to a predetermined cross-correlation threshold. The computer program further comprises determining that the predetermined cross-correlation threshold is less than approximately 0.2 for an orthogonal frequency domain multiplexing (OFDM) computer program using a 1024 fast Fourier transform (FFT) mode. In another feature, the computer program further comprises generating each bit of one of the derived preamble sequences having a first state when a corresponding bit and a bit adjacent to the corresponding bit in a corresponding one of the preamble sequences have opposite states, and generating the each bit having a second state when the corresponding bit and the bit adjacent to the corresponding bit have the same state. In another feature, the computer program further comprises storing the derived preamble sequences. In another feature, the computer program further comprises differentially demodulating the modulated signals having a fractional carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. In another feature, the computer program further comprises differentially demodulating the modulated signals having an integer carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. The computer program further comprises correlating the derived preamble sequences with the differentially demodulated signals having the integer CFO. In another feature, the computer program further comprises differentially demodulating the modulated signals having a linearly increasing phase offset generated by a symbol timing offset. In another feature, the computer program further comprises calculating a symbol timing offset by multiplying a phase angle of the largest correlation value by (N/2πP), where N is a number of sub-carriers in an N fast Fourier transform (FFT) mode, and where N is an integer greater than 1. N is one of 128, 512, 1024, and 2048. In still other features, a system comprises a correlation module and a control module. The correlation module correlates modulated signals with a plurality of preamble sequences and generates correlation values. The control module selects a largest correlation value from the correlation values and detects one of the preamble sequences in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a first predetermined threshold. The first predetermined threshold is based on the signal strength of the modulated signals. In another feature, each of the preamble sequences is different from others of the preamble sequences. The preamble sequences are stored in one of the correlation module and the control module. In another feature, the control module identifies a segment of a base station that transmitted the modulated signals based on the one of the preamble sequences. In another feature, the modulated signals include sub-carriers that are modulated using orthogonal frequency domain multiplexing (OFDM). The sub-carriers have a common channel gain and a random channel phase. Every P In another feature, the preamble sequences have a cross-correlation value of less than or equal to a second predetermined threshold. The second predetermined threshold is less than approximately 0.2 for an orthogonal frequency domain multiplexing (OFDM) system using a 1024 fast Fourier transform (FFT) mode. In another feature, the modulated signals include a fractional carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. In another feature, the modulated signals include an integer carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. In another feature, a physical layer module (PHY) comprises the system and further comprises a transceiver module that communicates with the correlation module and the control module and that receives the modulated signals. In another feature, a network device comprises the PHY and further comprises at least one antenna that communicates with the transceiver module. In another feature, the control module divides N of the sub-carriers into L bands when a channel gain of the sub-carriers is not substantially the same for all of the sub-carriers, where N and L are integers greater than or equal to 1, and where each of the L bands includes N/L of the sub-carriers. In another feature, the correlation module correlates symbols in every P In another feature, the control module generates a band correlation value for each of the L bands and for each of the preamble sequences by adding the intra-band correlation values. The control module further generates a magnitude of each of the band correlation value. The control module further generates the correlation values by adding the magnitude of each of the band correlation value for each of the preamble sequences. In still other features, a method comprises correlating modulated signals with a plurality of preamble sequences, generating correlation values based on the correlating, selecting a largest correlation value from the correlation values, and detecting one of the preamble sequences in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a first predetermined threshold. The method further comprises determining the first predetermined threshold based on the signal strength of the modulated signals. In another feature, the method further comprises storing the preamble sequences, wherein each of the preamble sequences is different from others of the preamble sequences. In another feature, the method further comprises identifying a segment of a base station that transmitted the modulated signals based on the one of the preamble sequences. In another feature, the method further comprises receiving the modulated signals that include sub-carriers that are modulated using orthogonal frequency domain multiplexing (OFDM), wherein every P In another feature, the method further comprises correlating the modulated signals with the preamble sequences having a cross-correlation value of less than or equal to a second predetermined threshold. The method further comprises determining that the second predetermined threshold is less than approximately 0.2 for an orthogonal frequency domain multiplexing (OFDM) method using a 1024 fast Fourier transform (FFT) mode. In another feature, the method further comprises receiving the modulated signals having a fractional carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. In another feature, the method further comprises receiving the modulated signals having an integer carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. In another feature, the method further comprises dividing N of the sub-carriers into L bands when a channel gain of the sub-carriers is not substantially the same for all of the sub-carriers, where N and L are integers greater than or equal to 1, and where each of the L bands includes N/L of the sub-carriers. In another feature, the method further comprises correlating symbols in every P In another feature, the method further comprises generating a band correlation value for each of the L bands and for each of the preamble sequences by adding the intra-band correlation values, generating a magnitude of each of the band correlation value, and generating the correlation values by adding the magnitude of each of the band correlation value for each of the preamble sequences. In still other features, a system comprises correlation means for correlating modulated signals with a plurality of preamble sequences and generating correlation values. The system further comprises control means for selecting a largest correlation value from the correlation values and detecting one of the preamble sequences in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a first predetermined threshold. The first predetermined threshold is based on the signal strength of the modulated signals. In another feature, each of the preamble sequences is different from others of the preamble sequences. The preamble sequences are stored in one of the correlation means and the control means. In another feature, the control means identifies a segment of a base station that transmitted the modulated signals based on the one of the preamble sequences. In another feature, the modulated signals include sub-carriers that are modulated using orthogonal frequency domain multiplexing (OFDM). The sub-carriers have a common channel gain and a random channel phase. Every P In another feature, the preamble sequences have a cross-correlation value of less than or equal to a second predetermined threshold. The second predetermined threshold is less than approximately 0.2 for an orthogonal frequency domain multiplexing (OFDM) system using a 1024 fast Fourier transform (FFT) mode. In another feature, the modulated signals include an integer carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. In another feature, a physical layer means (PHY) for communicating comprises the system and further comprises transceiver means for communicating with the correlation means and the control means and that receives the modulated signals. In another feature, a network device comprises the PHY means and further comprises at least one antenna means for communicating with the transceiver means. In another feature, the control means divides N of the sub-carriers into L bands when a channel gain of the sub-carriers is not substantially the same for all of the sub-carriers, where N and L are integers greater than or equal to 1, and where each of the L bands includes N/L of the sub-carriers. In another feature, the correlation means correlates symbols in every P In another feature, the control means generates a band correlation value for each of the L bands and for each of the preamble sequences by adding the intra-band correlation values. The control means further generates a magnitude of each of the band correlation value. The control means further generates the correlation values by adding the magnitude of each of the band correlation value for each of the preamble sequences. In still other features, a computer program executed by a processor comprises correlating modulated signals with a plurality of preamble sequences, generating correlation values based on the correlating, selecting a largest correlation value from the correlation values, and detecting one of the preamble sequences in the modulated signals upon determining that a magnitude of the largest correlation value is greater than or equal to a first predetermined threshold. The computer program further comprises determining the first predetermined threshold based on the signal strength of the modulated signals. In another feature, the computer program further comprises storing the preamble sequences, wherein each of the preamble sequences is different from others of the preamble sequences. In another feature, the computer program further comprises identifying a segment of a base station that transmitted the modulated signals based on the one of the preamble sequences. In another feature, the computer program further comprises receiving the modulated signals that include sub-carriers that are modulated using orthogonal frequency domain multiplexing (OFDM), wherein every P In another feature, the computer program further comprises correlating the modulated signals with the preamble sequences having a cross-correlation value of less than or equal to a second predetermined threshold. The computer program further comprises determining that the second predetermined threshold is less than approximately 0.2 for an orthogonal frequency domain multiplexing (OFDM) computer program using a 1024 fast Fourier transform (FFT) mode. In another feature, the computer program further comprises receiving the modulated signals having a fractional carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. In another feature, the computer program further comprises receiving the modulated signals having an integer carrier frequency offset (CFO) that generates a phase error that is substantially the same in each one of the modulated signals. In another feature, the computer program further comprises dividing N of the sub-carriers into L bands when a channel gain of the sub-carriers is not substantially the same for all of the sub-carriers, where N and L are integers greater than or equal to 1, and where each of the L bands includes N/L of the sub-carriers. In another feature, the computer program further comprises correlating symbols in every P In another feature, the computer program further comprises generating a band correlation value for each of the L bands and for each of the preamble sequences by adding the intra-band correlation values, generating a magnitude of each of the band correlation value, and generating the correlation values by adding the magnitude of each of the band correlation value for each of the preamble sequences. In still other features, the systems and methods described above are implemented by a computer program executed by one or more processors. The computer program can reside on a computer readable medium such as but not limited to memory, non-volatile data storage and/or other suitable tangible storage mediums. Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the disclosure, are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the term module, circuit and/or device refers to an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable hardware components that provide the described functionality. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. Referring now to Specifically, each BS may transmit data in three segments: SEG When a receiver in the MS is turned on (i.e., when the MS is powered up), the MS may associate with an appropriate segment of a corresponding BS depending on the location of the MS. The MS, however, can process data in a frame transmitted by a BS only if the MS can correctly detect a preamble sequence in the frame. Specifically, the MS can perform frame synchronization and retrieval of a cell ID (IDcell) and a segment number of the BS from the frame if the MS can detect the preamble sequence in the frame. Referring now to A total of 114 preamble sequences exist for OFDMA systems that use fast Fourier transforms (FFT) to modulate 1024 and 512 sub-carriers. Each preamble sequence is unique. That is, each preamble sequence is distinct from another preamble sequence and is identified by an index number. The index number may be referred to as preamble sequence index. Each preamble sequence is 284 and 143 bits long for 1024 and 512 FFT modes, respectively. Since one MS may typically communicate with up to three base stations, each BS modulates every third sub-carrier. That is, each BS modulates one of every three sub-carriers. Additionally, each BS uses only one bit of the total bits in a preamble sequence when modulating every third sub-carrier. For example, in 1024 FFT mode, the BS may use bit numbers Each BS may use the same set of sub-carriers. Each segment in a BS, however, uses distinct sub-carriers at least for preamble purposes. For example, for each BS, segment Consequently, the MS receives distinct signals from each BS. For example, the MS may receive signals from SEG A set of sub-carriers for segment n may be mathematically expressed as follows.
Typically, when the receiver in the MS is turned on, the MS initially performs symbol timing and carrier frequency synchronization before the MS can detect a preamble sequence. The MS may perform these tasks using a cyclic prefix in the data frame. Thereafter, the MS determines whether a first symbol in the frame is a preamble symbol. If the first symbol is a preamble symbol, then the MS determines which preamble sequence is present in the frame. Once the MS determines the preamble sequence, the MS can associate with a corresponding segment of an appropriate BS. Symbols in preamble sequences (i.e., preamble symbols) typically have higher energy than data symbols. For example, the energy of the preamble symbols is typically 8/3 times (i.e., 4.26 dB higher than) the energy of data symbols. This is useful in distinguishing preamble symbols from data symbols. Additionally, the preamble sequences are almost orthogonal. That is, a cross-correlation between any two preamble sequences is very small. For example, the cross-correlation is typically less than 0.2. This is useful in distinguishing individual preamble sequences from one another. As shown in the table in Referring now to When a preamble bit (i.e., a preamble symbol) in a preamble sequence is 0, the corresponding transmit signal Xi[k] is 1. When a preamble bit in a preamble sequence is 1, the corresponding transmit signal Xi[k] is −1. That is, when a preamble bit in a preamble sequence is 1, the phase of the sub-carrier in the transmit signal Xi[k] is shifted by π relative to the phase of the sub-carrier when a preamble bit in a preamble sequence is 0. The system When all k sub-carriers have a common channel gain H (i.e., when H[k] is independent of k) and when all k sub-carriers have random channel phase, the k sub-carriers are referred to as “almost flat frequency channels.” For almost flat frequency channels, the input signal may be mathematically expressed as follows.
A cross-correlation between different preamble sequences is given by the following formula. Since the cross-correlation between different preamble sequences is small, the correlation module The correlation module
The control module Thereafter, the control module The control module Thus, when the control module Occasionally, the input signal may comprise a carrier frequency offset (CFO). The CFO may be fractional or integer. An input signal comprising fractional CFO may be mathematically expressed as follows.
Additionally, the fractional CFO decreases signal to noise ratio (SNR) of the input signal. This is mathematically expressed as follows. Since fractional CFO introduces ICI and attenuates the input signal, the fractional CFO adversely affects preamble sequence detection in system On the other hand, when the CFO is an integer I, a phase error θ introduced by the integer CFO may be common to all k sub-carriers. In that case, the input signal may be mathematically expressed as follows.
Specifically, the integer CFO causes a cyclic shift of the input signal in the frequency domain. In other words, the integer CFO rotates the input signal in the frequency domain. Accordingly, the correlation module
Consequently, the control module
Occasionally, the channel may not be almost frequency flat. That is, the sub-carriers in a channel may not have a common channel gain H. In other words, H[k] may not be independent of k. In that case, the channel gain may vary with frequency of sub-carriers as shown in However, the system Assuming that the channel gain does not vary significantly among the sub-carriers within individual bands although the channel gain varies across the bands, the correlation module Thus for a preamble sequence, the control module The control module Referring now to Adjacent modulated sub-carriers (i.e., sub-carriers On the other hand, when adjacent modulated sub-carriers have an unknown differential channel phase that is common to all k sub-carriers, the channel phase difference between adjacent modulated sub-carriers may be non-zero. When adjacent modulated sub-carriers have similar channel phase or unknown differential channel phase common to all sub-carriers, the sub-carriers are generally referred to as “moderately frequency selective channels.” The differential demodulation module When the adjacent modulated sub-carriers have similar channel phase, the differentially demodulated signal can be mathematically expressed as follows.
The correlation module A cross-correlation between the derived preamble sequences is given by the following formula. Since the cross-correlation between the derived preamble sequences is small, the correlation module The correlation module The control module On the other hand, when the adjacent modulated sub-carriers have an unknown differential channel phase θ that is common to all k sub-carriers, the demodulated signal generated by the demodulation module Since the cross-correlation between the derived preamble sequences is small, the correlation module
The control module As in system
Occasionally, the input signal may have a small symbol timing offset due to improper symbol timing synchronization, which is performed when the MS is powered up. The symbol timing offset may cause inter-symbol interference (ISI). Additionally, the symbol timing offset may cause an inter-carrier interference (ICI). The input signal having a symbol timing offset can be mathematically expressed as follows. Specifically, the symbol timing offset introduces an extra phase offset among the sub-carriers. The phase offset may increase linearly as the sub-carrier index k increases. In that case, system The differentially demodulated signal with the extra phase offset is mathematically expressed as follows. In system Referring now to The control module If true, however, the control module If, however, the result of step The control module Referring now to A control module The control module If true, however, the control module Referring now to A control module The control module If true, however, the control module Although every third sub-carrier is modulated as described in the systems and methods disclosed in this disclosure, skilled artisans can appreciate that the systems and methods disclosed herein may be implemented by modulating every P Referring now to Referring now to The vehicle control system The power supply Referring now to The phone control module Memory Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims. Patent Citations
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